Multi-wavelength laser devices are widely used in modern wavelength division multiplexing (WDM) optical communication systems. A single-chip integrated multi-wavelength laser array has a wide application prospect in the WDM systems due to its compact size, low loss in both optical and electrical connections, high stability, and high reliability. There have been developed a number of methods for manufacturing the single-chip integrated multi-wavelength laser array, including electron-beam lithography, multi-step holographic exposure, sampling grating, and selective area epitaxial growth (SAG), etc.
Among these methods, fabrication of the single-chip integrated multi-wavelength laser array using the SAG technique has attracted increasing attention due to its simple process. An existing solution for manufacturing the single-chip integrated multi-wavelength laser array using the SAG technique includes, inter alia, forming an SAG mask pattern on a substrate; SAG of active-region materials, which consist a top separate confinement layer (SCL), a bottom SCL, and a multi-quantum-well layer; forming a grating on the top SCL; and growth of a contact layer. According to this solution, the thickness of the active-region materials can be varied using the SAG so that the materials for different laser units in the array may have different effective diffraction indexes, which in turn results in different lasing wavelengths for the different laser units. A drawback of this solution is that in addition to the effective refractive indexes of the materials, emission wavelengths of the quantum wells also vary with the thickness of the materials. The emission wavelength of the quantum-well material is sensitive to its thickness, causing the variation of the emission wavelengths of the quantum well with the quantum well thickness to be much more rapid than Bragg wavelengths of the DFB lasers. As a result, the Bragg wavelengths of some laser units will deviate from the peak gain value of the quantum-well material, thereby deteriorating its single-mode characteristic. Also, quality of the quantum-well material may degrade seriously if the thickness thereof is excessively increased because the quantum-well material typically has a large strain.